The trick to using air dry clay under the polymer clay is the air dry clay has to be bone dry and sealed before you cover it. The polymer clay will crack if the air dry clay is not completely dry. I use the air dry as the armature base for my pen holders. I have tried several brands and the one that has work best for me is the Crayola air dry clay. it also seems to shrink less then the other brands. I will let my air dry clay basis dry for at least 25/30 days. Then apply a thin layer of Sobo glue and let set again for at least 2 weeks. When you cover the air dry clay with the polymer leave a couple of small holes. The holes will allow any moisture that is still in the air dry clay to escape.
You can see my pen bases here http://www.flickr.com/photos/mrsezy/. They are in the 2112 set.